Measurements are provided in a patient of the functioning of various organs in a patient's body. For example, measurements are made of the functioning of the patient's heart and the patient's brain. These measurements are generally made by applying an electrode or electrodes to the skin of the patient at the appropriate position or positions in the patient's body and by evaluating the characteristics of the signal produced at the particular position or positions.
The measurements of the functioning of different organs in the patient's body involve different frequency ranges. For example, measurements of the patient's heart occur in a range of DC to approximately two hundred fifty hertz (250 Hz) and measurements of the patient's brain occur in a range of DC to approximately one hundred and fifty hertz (150 Hz).
The measurement of the functioning of different organs in the patient's body involves acquiring signals of miniscule amplitudes. For example, the range of voltages produced at an electrode attached to the patient's skin for a measurement of the patient's heart is approximately one-half of a millivolt (0.5 mV) to approximately four millivolts (4 mV). The range of voltages produced at an electrode attached to the patient's skin for a measurement of the patient's brain is approximately five microvolts (5 μV) to approximately three hundred microvolts (300 μV).
When an electrode is attached to the patient's skin to measure the function of an organ such as the patient's heart or brain, the bioelectric signal generated from the organ has to penetrate from the patient's organ through the body to the patient's skin and to the electrode attached to the patient's skin. The patient's skin has many layers. The greater the number of layers that the signal has to penetrate in the patient's skin, the greater is the impedance and barrier that the skin presents to the signal generated by the organ whose function is being measured. The problem of high impedances is compounded if the patient's skin is not clean and prepared by abrading when the measurement is being made. Thus, the impedance presented by the patient's skin may be as high as approximately several thousand ohms to approximately several hundred thousand ohms. The input, impedance of the recording amplifier connected to the electrode is preferably very high so as not to alter the characteristics of the original signal.